1. Field of the Invention
The present invention relates to a projection exposure apparatus and method in which highly accurate imaging characteristics are required to manufacture semiconductor integrated circuits, liquid crystal devices, thin film magnetic heads, etc. and more particularly to the maintenance of imaging characteristic performance of a projection optical system.
2. Related Background Art
In a photolithography process for forming a circuit pattern of a semiconductor device, etc., a circuit pattern formed on a reticle (mask) is transferred to a substrate (a semiconductor wafer, glass plate, etc.). More specifically, photosensitive photoresist is applied to the substrate and the pattern of the reticle is transferred to the photoresist in accordance with transparent portions of the pattern of the reticle. In a projection exposure apparatus (e.g., stepper), the image of a reticle pattern is transferred to a wafer via a projection optical system.
In such an apparatus, light flux from a light source is made into approximately a round cross-sectional shape (or rectangular shape) with the optical axis of an illumination optical system as the center in a plane of the illumination optical system (hereinafter called a pupil plane of the illumination optical system) being a Fourier transform plane with respect to the pattern of the reticle, or in a plane close to the pupil plane of the illumination optical system. Therefore, the light flux is incident on the reticle approximately perpendicularly. Also, the circuit pattern of the reticle (glass plate such as quartz) is constituted of light-transmitting portions with the transmittance of about 100% with respect to the light flux and light-shielding portions with the transmittance of about 0%.
In order to transfer a more minute pattern to the wafer in the conventional exposure method, it is necessary to use a light source for emitting light with a much shorter wavelength or to use a projection optical system with a much larger numerical aperture. However, presently, it is difficult to make the light source emit light with a much shorter wavelength (200 nm or less) since there is no optical material usable as an optical transmitting member and there is not a stabilized light source capable of emitting a large quantity of light. Also, the increase of the numerical aperture of the projection optical system reaches the theoretical limitation and further increase is hopeless now. Even if the increase of the numerical aperture were achieved, the depth of focus determined by .+-..lambda./NA.sup.2 is decreased in accordance with the increase of the numerical aperture, so that the depth of focus is shorted and then a practical exposure apparatus cannot be obtained.
Then, a phase shift reticle provided with a phase shifter (dielectric thin film, etc.) has been proposed in which the phase of the light transmitted through a specific light-transmitting portion among the light-transmitting portions is shifted .pi. (rad) with respect to the light transmitted through the other light-transmitting portions. Such a phase shift reticle is disclosed in, e.g., Japanese Patent Publication No. 62-50811. According to the phase shift reticle, the transfer of a more minute pattern is possible as compared with the ordinary reticle (the conventional reticle with the light-transmitting portions and the light-shielding portions). That is, there is an effect of improving the resolving power. When using the phase shift reticle, the numerical aperture (coherence factor .sigma.) of the illumination optical system needs to be optimized. Although so far, various types of phase shift reticles have been proposed, typical ones are a spatial frequency modulation type (Japanese Patent Publication No. 62-50811), a half tone type (Japanese Patent Application Laid-Open No. 4-162039), a shifter light-shielding type and an edge emphasizing type.
Also, recently, an attempt has been made to optimize the illumination condition or to enable the transfer of a minute pattern by finding a suitable exposure method. For example, in U.S. Pat. No. 4,931,830, a method has been proposed to improve the resolving power and the depth of focus by selecting a combination of the numerical aperture (.sigma. value) of an illumination optical system and the numerical aperture of a projection optical system for each pattern line width of patterns. Further, an annular illumination method (Japanese Patent Application Laid-Open No. 61-91662) and a deformation light source method or an inclining illumination method (Japanese Patent Application Laid-Open No. 4-101148, and Japanese Patent Application No. 4-408096) have been proposed. In the annular illumination method, the light quantity distribution of light flux from a light source is defined in an annular shape in the pupil plane of an illumination optical system or in a plane close to the pupil plane. In the inclining illumination method, the light quantity distribution of light flux from a light source in the pupil plane of an illumination optical system, or in a plane close to the pupil plane is made maximum at least one position being off-centered a predetermined length from the optical axis of the illumination optical system and so the light flux is inclined a predetermined angle with respect to the reticle pattern. However, either method is not effective for all reticle patterns, i.e., all pattern line widths and shapes, and needs to select an optimum illumination method and an illumination condition for each reticle or each pattern of reticles. Therefore, it is necessary to provide a projection exposure apparatus with a structure for varying the illumination condition (.sigma. value) in the illumination optical system.
In projection exposure apparatuses, recently, there have been strong demands on maintaining imaging characteristics (magnification, focus position) of the projection optical system to be a predetermined value with high accuracy. Therefore, various methods for correcting imaging characteristics have been proposed and put to practical use. Among those methods, a method for correcting the change of imaging characteristics due to light absorption of the projection optical system is disclosed in, e.g., U.S. Pat. No. 4,666,273. In this method, the amount of stored energy (amount of heat) in the projection optical system due to the incidence of light (i-lines, KrF excimer laser, etc.) to the projection optical system is sequentially calculated, and the amount of imaging characteristic change due to the amount of stored energy is obtained, whereby the imaging characteristics are adjusted finely by a predetermined correcting mechanism. As the correcting mechanism, for example, there is a method in which a space between two lens elements among a plurality of lens elements constituting the projection optical system is sealed and the pressure of the sealed space is regulated.
When changing the illumination condition of the illumination optical system as above, or when exchanging the reticle, or when switching the pattern within the reticle by driving a reticle blind (illumination field stop), the distribution of the quantity of transmitted light is changeable in the pupil plane of the projection optical system or in lens elements close to the pupil plane due to the above change. As light passes an area in the vicinity of the pupil plane concentrically, when the light quantity distribution is changed in there, the change of imaging characteristics due to light absorption of the projection optical system is largely influenced by that matter. Then, as disclosed in, e.g., Japanese Patent Application Laid-Open No. 62-229838, it is conceivable that calculation parameters used for calculating the amount of imaging characteristic change of the projection optical system due to the light absorption are corrected (exchanged) for each illumination condition, and the change of imaging characteristics due to the changing of illumination condition is obtained accurately by the use of the corrected parameters (i.e., calculation parameters most suitable for the illumination condition after the changing operation), and is corrected. Also, even though the projection optical system is not affected by the light absorption, the imaging characteristics might be changed by fine aberration conditions of the projection optical system due to the differences between passing positions of light flux. Therefore, a method for imparting a predetermined offset to the correction value of the imaging characteristics in accordance with the change of the illumination condition has been considered.
When correcting the imaging characteristics by the use of the correction parameters, there is no problem from the standpoint of a long period of time. However, the phenomenon of the heat storage of the projection optical system has past hysteresis. Therefore, when changing the illumination condition corresponding to the reticle and its pattern, even though the amount of imaging characteristic change is immediately calculated and corrected in accordance with calculation parameters corrected under a new illumination condition, inconveniences occur for correcting the imaging characteristics since the hysteresis corresponding to the illumination condition set prior to its changing operation is left, and this hysteresis is not taken into consideration when only the calculation parameters are corrected.
The first inconvenience is that due to the heat distribution produced by the previous illumination condition, even though the above-mentioned offset component is taken into consideration, the calculated imaging characteristics under the new illumination condition (after the changing of the illumination condition) does not coincide with the actual imaging characteristics. Namely, as the offset component is obtained in the state that the projection optical system is not affected by light absorption, if the influence of the light absorption in the previous illumination condition is left, an offset for the influence needs to be imparted further. That is, the amount of imaging characteristic change becomes discontinuous before and after the changing of the illumination condition. Therefore, the imaging characteristics cannot be corrected continuously to the changing of the illumination condition.
The second inconvenience occurs at the time of performing exposure under the new illumination condition even though the first inconvenience is solved by any method. That is, immediately after the changing of the illumination condition, in the lens elements close to the pupil plane of the projection optical system, the heat distribution condition under the previous illumination condition and the heat distribution condition under the new illumination condition are superimposed to be a heat distribution different from both states. Therefore, even though the amount of imaging characteristic change is calculated by the calculation parameters under either illumination condition, the result of the calculation will not coincide with the actual amount of imaging characteristic change. The imaging characteristics (i.e., the heat distribution condition of the projection optical system) in such a transient state cannot be expressed by the simple sum of both. Therefore, in this transient state, it is very difficult to calculate and correct the amount of imaging characteristic change precisely. Accordingly, when a pattern exposure is carried out to a wafer under the transient state, it is impossible to obtain a circuit pattern satisfying the initial characteristics.
Then, a method for solving such a problem has been proposed in, e.g., Japanese Patent Application Laid-Open No. 6-45217. In this method, when the illumination condition corresponding to a reticle or its pattern is changed, an exposure operation with respect to a photosensitive substrate is stopped until the amount of imaging characteristic change due to light absorption of a projection optical system under the previous illumination condition becomes a predetermined permissible value or less, or the influence of the amount of energy (hysteresis) stored in the projection optical system prior to the change of the illumination condition with respect to the imaging characteristics is decreased to a negligible degree. Thereafter, the exposure operation is performed while the imaging characteristics of the projection optical system are controlled under the new illumination condition. According to this method, in the discontinuous or transient condition at the time of the changing the illumination condition, the exposure operation is not performed, so that the imaging characteristics of the projection optical system can be controlled precisely for each illumination condition. However, the exposure operation should be stopped each time the illumination condition or the reticle pattern is changed (each time the light quantity distribution in the pupil plane of the projection optical system is changed). Therefore, the throughput of the exposure apparatus is lowered considerably and this method cannot be said to be practical.
Also, in the transient condition after the changing of the illumination condition, as disclosed in, e.g., Japanese Patent Publication No. 3-40934, and Japanese Patent Application Laid-Open No. 4-348019, it is considered that the imaging characteristics of the projection optical system are consequently measured by the use of a reference mark on a wafer stage and exposure is executed while the imaging characteristics are corrected properly based on the result of the measurement. However, in order to measure the imaging characteristics precisely, the light quantity distribution in the pupil plane of light coming from the reference mark on the wafer state and passing the projection optical system should be made to coincide with the light quantity distribution in the pupil plane of light for exposure passing the projection optical system at the time of exposure. But, in this case, measuring means (mark detecting system) becomes complex and large in size. Further, the imaging characteristics need to be measured by interrupting the exposure operation in this method too, so that the lowering of the throughput of the exposure apparatus cannot be avoided.